This invention relates to the preparation of continuously variable-composition copolymers by effecting gradual changes in monomer composition during the polymerization process. An example of application of this process is the preparation of poly(meth)acrylate copolymers that have improved lubricating oil additive properties, for example, as pour point depressants or viscosity index improvers, when compared to related polymer additives made by conventional means.
The behavior of petroleum oil formulations under cold flow conditions is greatly influenced by the presence of paraffins (waxy materials) that crystallize out of the oil upon cooling; these paraffins significantly reduce the fluidity of the oils at low temperature conditions. Polymeric flow improvers, known as pour point depressants, have been developed to effectively reduce the "pour point" or solidifying point of oils under specified conditions (that is, the lowest temperature at which the formulated oil remains fluid). Pour point depressants are effective at very low concentrations, for example, between 0.05 and 1 percent by weight in the oil. It is believed that the pour point depressant material incorporates itself into the growing paraffin crystal structure, effectively hindering further growth of the crystals and the formation of extended crystal agglomerates, thus allowing the oil to remain fluid at lower temperatures than otherwise would be possible.
One limitation of the use of pour point depressant polymers is that petroleum base oils from different sources contain varying types of waxy or paraffin materials and not all polymeric pour point depressants are equally effective in reducing the pour point of different petroleum oils, that is, a polymeric pour point depressant may be effective for one type of oil and ineffective for another. It would be desirable for a single pour point depressant polymer to be useful in a wide variety of petroleum oils.
One approach to solving this problem is disclosed in "Depression Effect of Mixed Pour Point Depressants for Crude Oil" by B. Zhao, J. Shenyang, Inst. Chem. Tech., 8(3), 228-230 (1994), where improved pour point performance on two different crude oil samples was obtained by using a physical mixture of two different conventional pour point depressants when compared to using the pour point depressants individually in the oils. Similarly, U.S. Pat. No. 5,281,329 and European Patent Application EP 140,274 disclose the use of physical mixtures of different polymeric additives to achieve improved pour point properties when compared to using each polymer additive alone in lubricating oils.
U.S. Pat. No. 4,048,413 discloses a process for the preparation of uniform-composition copolymers by controlling the ratio and rate of addition of the monomers added to a polymerizing mixture of the monomers to offset the natural differences in reactivities of the individual monomers that would normally lead to compositional "drift" during conventional polymerizations. There is no disclosure in U.S. Pat. No. 4,048,413 of controlling the ratio and rate of addition of monomers to a polymerization mixture to provide a continuously changing- or continuously variable-composition copolymer.
None of these previous approaches provides good low temperature fluidity when a single polymer additive is used in a wide range of lubricating oil formulations. It is an object of the present invention to provide a process for preparing copolymers having a continuously variable-composition and, in doing so, to also provide polymers having the aforementioned desired combination of lubricating oil properties in a single polymer additive.